1. Field of the Invention
The present invention relates to a multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, and a method for determining the bit-loading in a multi-carrier transmission system.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
Modern multi-carrier techniques using orthogonal carriers with high order QAM constellations for the transmission of a plurality of bits per carrier and symbol use some method for determining a suitable constellation (bit-loading) for each separate carrier. This is usually based on an estimate of the signal-to-noise ratio (SNR) of each carrier and the maximum allowed symbol error rate.
It is an object of the present invention to provide, in a multi-carrier transmission system, a simplified method for determining bit-loading which exploits the fact that all signals are normalised to the same scale at a symbol detector input and output.
It is another object of the present invention to provide a multi-carrier transmission system in which a suitable constellation (bit-loading) is determined for each carrier, the bit-loading determination exploiting the fact that all signals are normalised to the same scale at a symbol detector input and output.
According to a first aspect of the present invention, there is provided a multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, said system including digital receiver and transmitter units, said receiver unit including a symbol detection unit, characterised in that said system is adapted to determine a parameter for each single carrier, said parameter being indicative of a deviation of a received signal from a corresponding constellation point; to compare the parameter with an upper and lower limit; and, if the parameter is outside said limits, to change the constellation used to modulate the carrier to a neighbouring constellation. The symbol detection unit may be used to determine the parameter for a carrier.
Said parameter may be a ratio d2/"sgr"2, where d is the shortest distance between neighbouring constellations, "sgr" is a standard deviation, and "sgr"2 is the variance of the deviations of the input and output signal values of said symbol detection unit.
The symbol detection unit may be adapted to associate an input signal applied thereto, in a range around a specific constellation point, to a distinct value of this point, said distinct value being the value of the output of said symbol detection unit, and to measure differences between said input and output signals for the carrier, said differences being squared and averaged to provide an estimate of a value for a which is represented in the same units as the squared distance d2 for the constellation used to modulate the carrier.
The differences between the input and output signal values of said symbol detection unit may vary in value, the variance being a measure of the disturbance on the carrier and directly related to the SNR. The variance may be proportional to the SNR.
An upper limit of said variance for a specific constellation may be determined by a maximum allowable symbol error rate and a lower limit for said variance may be equal to an upper limit for the next largest constellation.
A specific maximum symbol error rate may give a minimum ratio of d/"sgr".
According to a second aspect of the present invention, there is provided, in a multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, said system including digital receiver and transmitter units, said receiver unit including a symbol detection unit, a method for determining a constellation (bit-loading) for each single carrier, characterised by the steps of determining a parameter for each single carrier, said parameter being indicative of a deviation of a received signal from a corresponding constellation point; comparing the parameter with an upper and lower limit; and, if the parameter is outside said limits, changing the constellation for the carrier to a neighbouring constellation.
In a preferred method, determination of the parameter for each single carrier is effected by said symbol detection unit, and the parameter is a ratio d2/"sgr"2, where d is the shortest distance between neighbouring constellations, a is a standard deviation, and "sgr"2 is the variance of the deviations of the input and output signal values of said symbol detection unit.
In accordance with a preferred method, determination of the parameter for a carrier includes the steps of associating an input signal to said symbol detection unit, in a range around a specific constellation point, to a distinct value of this point, said distinct value being the value of the output of said symbol detection unit; and measuring differences between said input and output signals of said symbol detection unit for the carrier, said differences being squared and averaged to provide an estimate of a value for d2 which is represented in the same units as the squared distance d2 for the constellation used to modulate the carrier. The differences between the input and output signal values of said symbol detection unit may vary in value, said variance being a measure of the disturbance on the carrier and directly related to the SNR. The variance may be proportional to the SNR.
In accordance with a preferred method, an upper limit of said variance for a specific constellation is determined by a maximum allowable symbol error rate and in that a lower limit for said variance is equal to an upper limit for the next largest constellation. A specific maximum symbol error rate may give a minimum ratio of d/"sgr".
According to a third aspect of the present invention, there is provided, a multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, said system including digital receiver and transmitter units, said receiver unit including a symbol detection unit, characterised in that said system uses a method as outlined in preceding paragraphs for determining a constellation or each single carrier.
The multi-carrier transmission system may be a DMT system, for example, a DMT based VDSL system.